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This implements an LSP Extension `PeekDocumentsRequest` to let `ExpandMacroCommand` to open the macro expansions in a "peeked" editor window.
For this to work, the client has to pass "workspace/peekDocuments" enabled to `ClientCapabilities.experimental` and the client should handle the `PeekDocumentsRequest` and show the expansions in a "peeked" editor window.
PR to support the above capability in the "Swift for VS Code" Extension: https://github.com/swiftlang/vscode-swift/pull/945
The "Swift for VS Code" extension cannot send the client capability, so it instead passes the same through `initializationOptions` in the `InitializeRequest`.
For editors which doesn't support this capability, `sourcekit-lsp` sends a `ShowDocumentRequest`.
The `ShowDocumentRequest` is updated to show all the macro expansions in a single generated file. Moreover, its folder structure is updated to use hex string of MD5 hash of concatenation of buffer names of expansions.
Fixes https://github.com/swiftlang/vscode-swift/issues/564
Fixes https://github.com/swiftlang/sourcekit-lsp/issues/1498 ( rdar://130207754 )
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